This invention relates to a magnetic chuck which can be changed over to retain a removable magnetic body, and more particularly to a magnetic chuck in which magnetic fluxes of permanent magnets are guided to a workpiece through magnetic materials to retain the workpiece and kept away from the workpiece to permit removal of the workpiece.
Magnetic chucks are well known in which a top plate having pole members of magnetic material alternating with separators of non-magnetic material is disposed above a base plate, i.e. magnet assembly having alternating yokes of magnetic material and permanent magnets. Magnetic fluxes from said permanent magnets of the magnet assembly are guided to a workpiece through the pole members of the top plate, magnetically holding the workpiece to the mounting surface of the top plate. By changing the position of said magnet assembly in relation to said top plate, the magnetic fluxes are diverted from the mounting surface and the workpiece can be removed. These well known magnetic chucks are of a type in which the fluxes are guided away from the workpiece or prevented from reaching it by separating the base plate or magnet assembly from the top plate and inducing magnetic lines of force to a bottom plate of magnetic material of the case receiving said magnet assembly. This diminishes the magnetic fluxes at the mounting surface of the top plate. Also known is a type of magnetic chuck in which the magnet assembly is slid laterally in relation to the top plate to change the paths of the loops of magnetic fluxes flowing from the permanent magnets. The present invention relates to the latter type of chuck.
Some magnetic chucks have auxiliary poles of magnetic members for inducing magnetic fluxes arranged between respective main pole members, and whereby thin magnetic poles are arranged on a surface of the magnetic chuck or top plate in order to hold thin workpieces or several small size magnetic parts firmly. In such magnetic chucks, when said magnetic assembly is slid laterally in relation to the top plate, the yokes of the base plate may not be oriented precisely in place depending on the accuracy of finishing of the respective main pole members, auxiliary pole members and separators and the accuracy of assembling the base plate poor finishing or assembly allows the magnetic fluxes to leak from the top plate to the workpiece to apply slight attraction when the magnetic assembly is shifted to its demagnetized position i.e. residual attraction thereto. This condition will be described with reference to FIG. 1. Permanent magnets 2 and yokes 1 are arranged alternatively to constitute a base plate, i.e. magnetic assembly 3. Opposed to this magnetic assembly 3 is disposed a top plate 7 having main pole members 4, auxiliary pole members 5 and separators 6 arranged to constitute a magnetic chuck together with a case, not shown. When the respective yokes 1 of the magnet assembly 3 are placed in the position shown by the solid lines in FIG. 1, i.e. opposed to the main pole members 4, the magnetic chuck is in magnetized condition. In this magnetized condition, fluxes 8 extend over the top plate 7 into workpiece 9, as shown in the drawing, and magnetically hold the workpiece 9. When the magnet assembly 3 is slid in the direction of arrow 10 to place the magnetic chuck in its the non-magnetized condition, the yokes 1 of the magnet assembly 3 are placed in the position shown by the phantom lines in FIG. 1, i.e. opposed to the auxiliary pole members so that magnetic fluxes 11 passing through the respective yokes 1 from loops. Consequently, the fluxes directed over the top plate 7 into the workpiece disappear, permitting the workpiece 9 to be removed. However, if the accuracy of finishing and assembling the respective parts is unsatisfactory, some yokes 1 of the base plate 3 may not be placed opposed to the center of the auxiliary pole member 5 such that b/a.noteq.1, where a and b respectively represent distances between either side of a yoke 1 and each auxiliary pole member 5 located on either side of the main pole member 4 corresponding to said yoke 1. Thus, magnetic fluxes 12 will leak over the top plate 7 into the workpiece 9 and cause residual attraction even in the non-magnetized condition.